Introduction.
X-rays are a very useful form of radiation to see into materials because most materials are quite transparent to x-rays: the complex refractive index at x-ray energies for most substances is very close to 1. Designing reflective and refractive optical elements analogous to those that are well known in the visible portion of the electromagnetic spectrum (where refractive indices are typically 1.4 or higher) cannot be used at x-ray wavelengths. Designing and constructing illuminators for applications of x-rays can therefore be particularly challenging.
For scientific studies of materials, where high brightness may be needed to obtain adequate signal-to-noise ratios over a range of x-ray energies, conventional x-ray sources using electron bombardment are simply not adequate.
For scientific studies of materials that need high brightness x-rays, and in particular the atomic structure and composition analysis that can be achieved by analyzing x-ray diffraction or fluorescence, high brightness synchrotrons or free-electron lasers have been used with great success. However, these facilities are large, often occupying acres of land, and expensive to operate, and obtaining beamtime can take months of waiting.
Laboratory systems that can be used for these applications, and in particular micro-x-ray fluorescence for materials analysis, would therefore be highly desired. The main problem for producing such a system is the lack of a suitable system with an x-ray source and efficient optics for achieving a tightly focused, high flux and high flux density x-rays.
X-Ray Fluorescence.
To better understand the utility of a high flux/high flux density x-ray illuminator, it helps to understand the requirements of the applications for which it will be used, and in particular, the requirements of x-ray fluorescence. When materials are exposed to high energy particles, such as x-rays and gamma rays, tightly held electrons from the inner electron shells of the atom can be ejected. To fill the vacancy so created, electrons in higher electron shells transition into the lower orbital, releasing energy difference between the electron shells in the form of an emitted photon. The energy level structure is distinct for each type of atom, and therefore the energy of the emitted photons is characteristic of the atoms present in the material. The term fluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the emission of lower energy radiation.
X-ray fluorescence is illustrated in FIG. 1, which shows a representation of the electrons around an atom with electrons in the K, L and M shells, and in which an incident high energy x-ray has ejected an electron from the inner K shell. When an electron from next higher shell (the L shell) transitions to fill the vacancy, the characteristic Kα1 x-ray photon for the material is emitted. When an electron from 2nd next higher shell (the M shell) transitions to fill the vacancy, the characteristic Kβ1 x-ray photon for the material is emitted. For most atoms, an empirical relationship called Moseley's Law relates the atomic number Z and the energy of the Kα1 fluorescence:EKα[keV]≈1.017×10−2(Z−1)2  [Eqn.1]In this manner, detection of the energy emitted indicates the presence of particular elements Z, and the strength of the fluorescence can be related to the relative concentrations of the atomic material.
FIG. 2 illustrates a simple conventional prior art x-ray fluorescence system P200. The system P200 comprises an x-ray source P80 comprising a high voltage supply P10, an electron emitter P11 that emits electrons P111 that bombard a target P100, generating x-rays P888. The x-rays P888 typically pass through a window P40 and irradiate a sample of material P240 held in a sample holder P244. Such a sample holder may be a simple tray, or comprise a complex mount, having controls for translation in x, y and z directions, and may also include x, y, and/or z-axis rotation mechanisms. A portion of the x-ray fluorescence P2888 emitted by the sample of material P240 is detected by a specially designed detector P290, which may be an electron drift detector that can discriminate between the energies of the x-ray photons detected, or may comprise a combination of a spectrometer and a detector, that generates an electronic signal representing the number of counts for the fluorescent x-rays at various energies. Once converted to electronic signals, various electronic components P292 may provide additional processing, and the data may be sent over a connector to an analysis system P295 for further analysis, which may also comprise a display P298.
Such systems are often employed in a lab environment, in which a sample of material is brought to the lab and mounted in the machine for analysis. With the reduction in size of modern electronics, XRF systems that are handheld have been developed. Such a system is illustrated in FIGS. 3A and 3B.
The handheld system H200 of FIGS. 3A and 3B also comprises a source H80 of x-rays H888 that are directed towards an object for analysis, in this example, a toy duck H240 being tested for the presence of toxic chemicals such as lead in its paint and materials. The x-ray fluorescence H2888 from the object H240 is detected by a specially designed detector H290 that generates an electronic signal representing the number of counts for the fluorescent x-rays at various energies. Once converted to electronic signals, various electronic components H292, optional digital signal processing components H293 and a central processing unit (CPU) H295 may provide additional processing and analysis of the signals, and presented on an integrated display H298 for the user or stored in integrated memory devices H299 for downloading and further analysis at a later time.
Microfocus XRF Systems.
Both of the prior art systems described so far simply illuminate a sample with x-rays and detect the fluorescence that is emitted from the illuminated area. However, for many applications, the atomic compositions of microscopic or even nanoscopic grains of material may be of interest. Therefore, additional prior art systems use a microfocus source of x-rays that can then be focused to a microscopic spot on the sample allow probing samples on a microscopic scale.
FIG. 4 illustrates the elements of a typical microfocus XRF system M200. The source M80 comprises a vacuum environment (typically 10−6 torr or better) commonly maintained by a sealed vacuum chamber M20 or active pumping, and manufactured with sealed electrical leads M21 and M22 that pass from the negative and positive terminals of a high voltage source M10 outside the tube to the various elements inside the vacuum chamber M20. The source M80 will typically comprise mounts M30 that secure the vacuum chamber M20 in a housing M50, and the housing M50 may additionally comprise shielding material, such as lead, to prevent x-rays from being radiated by the source M80 in unwanted directions.
Inside the chamber M20, an electron emitter M11 connected through the lead M21 to the high voltage source M10 serves as a cathode and generates a beam of electrons M111, often by running a current through a filament. A target M100 comprising a target substrate M110 and regions M700 of x-ray generating material is electrically connected to the opposite high voltage lead M22 and target support M32 to be at ground or relative positive voltage, thus serving as an anode. The electrons M111 accelerate towards the target M100 and collide with it at high energy, with the energy of the electrons determined by the magnitude of the accelerating voltage. The collision of the electrons M111 into the target M100 induces several effects, including the emission of x-rays 888, some of which are transmitted through a window M40 that is transparent to x-rays.
To create the microfocus x-ray spot on the target, an electron control mechanism M70 such as an electrostatic lens system or other system of electron optics that is controlled and coordinated with the electron dose and voltage provided by the electron emitter M11 by a controller M10-1 through a lead M27. The electron beam M111 may therefore be focused, and scanned onto the target M100.
Once the x-rays 888 exit the source M80, a portion of the x-rays are collected by a set of x-ray optics M840 that focus a portion 887 of the x-rays onto the sample 240 to be investigated. X-rays that are not collected and focused may be blocked by a beam stop M850. Once the focused portion of the x-rays 887 converge onto the sample 240, x-ray fluorescence photons 2888 will propagate away from the sample 240 onto a detector M290. As in the other prior art systems, the detector M290 converts the detected counts to electronic signals, which may be further processed by signal processing electronics M292 and passed to an analysis system M295.
X-ray fluorescence is a technique that can be applied to biomedical imaging, materials science, geological, and semiconductor applications and enable up to parts-per-billion sensitivity to map multiple trace elements. It provides several key advantages over charged-particle based techniques such as electron-based imaging (e.g. minimal sample preparation, near absence of a limiting bremsstrahlung background, and significantly reduced radiation damage) [see C. J. Sparks, “X-ray fluorescence microprobe for chemical analysis.” in Synchrotron Radiation Research (Springer Verlag-US, 1980), pp. 459-512] and complementary and unique capabilities compared to laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS) (e.g. better absolute detection limits, non-destructive, and sensitivity to non-metals) [see S. Vogt. “X-ray fluorescence microscopy: a tool for biology, life science and nanomedicine”, presentation posted online at:
commons.lib.jmu.edu/photon/2012/presentations/9/].
XRF analysis offers many inherent advantages for elemental analysis due to the unique interaction of x-rays with matter and the characteristic (signature) x-ray energies (lines) of each and every element in the periodic table with Z>3. The technique is nearly nondestructive, simultaneously detects multiple elements, and achieves high signal-to-background ratio, which leads to high sensitivity (low absolute and relative detection limit). In principle, x-ray fluorescence can theoretically realize single atom detection, similar to single molecule detection using light fluorescence techniques, as each atom can yield multiple characteristic fluorescence x-rays with continuous core shell ionization and de-excitation processes.
MicroXRF, in which x-rays are focused to areas with diameters of microns or tens of microns to achieve high-resolution imaging, has long been achieved using x-ray focusing optics and a synchrotron as the x-ray source. However, synchrotrons are large facilities, often taking up acres of land, and beam time is not available for routine analysis. Laboratory systems have been designed using similar x-ray optics, but typically cannot achieve the brightness or x-ray flux possible with synchrotron systems.
There are inherent advantages of XRF for trace level analysis at micron-scale resolution (microXRF) over other techniques for detecting atomic species, such as the dedicated electron microprobe analyzer (EMPA) and scanning electron microscope (SEM) with an x-ray analyzer. These advantages of x-ray induced XRF include:                (1) near absence of the broad bremsstrahlung x-ray background encountered in charged particle based techniques that limits sensitivity to several hundred parts per million;        (2) the significantly lower radiation damage;        (3) the need for minimal specimen preparation, leading to fewer artifacts and lower loss of volatile components; and        (4) the convenient sample environment (typically operating in ambient condition), with significantly increased ease of use.The perceived disadvantage of laboratory microXRF is that the excitation spot is too large (typically around 30 microns). For many applications, analysis of material compositions and structure on the micron or sub-micron scale is desired. The spot size is limited due to the low throughput at smaller spot sizes, caused by a combination of low flux at the sample and low solid angle of collection for the x-ray fluorescence.        
MicroXRF has complementary and unique capabilities when compared with alternative techniques for mapping elemental distributions such as laser ablation chemical analysis techniques including laser-ablation inductively-coupled-plasma mass spectroscopy technique (LA-ICPMS), which is widely adopted for mapping elemental distribution with a spatial resolution typically in the range of 50-100 micrometers. There are several outstanding reviews comparing XRF with this technique [see Z. Y. Qin et al. “Trace metal imaging with high spatial resolution: Applications in biomedicine.” Metallomics vol. 3 (2011), pp. 28-37; and R. Ortega et al. “Bio-metals imaging and speciation in cells using proton and synchrotron radiation xray microspectrometry.” Journal of the Royal Society Interface vol. 6 (2005) pp. S649-S658.]. Though LA-ICPMS generally offers lower (better) relative detection limit for metals with Z>30 and a unique ability to detect isotopes, it is destructive of the sample (via ablation), has an inferior absolute detection limit, and suffers from polyatomic interference of many elements with Z<30 for complex matrix materials, like biological specimens. To detect 1000 ions of a given element, a minimum of 108 atoms of the element are required as the input. Furthermore, the detection sensitivity (both absolute and relative) is highly compromised for non-metals (such as sulfur (S), phosphorous (P), and selenium (Se)) and especially halogens (such as fluorine (F), chlorine (Cl), or bromine (Br)) due to their low ionization cross-sections and polyatomic interference.
Due to the demand from the biomedical and materials science communities, a large number of scanning microXRF microprobes have been developed for use in synchrotron radiation facilities around the world with unprecedented capabilities, including parts per billion relative detection limit, 1000 atoms absolute detection limit, sub-50 nm resolution, and fly-scan techniques with sub-3 ms data collection per data point and up to a million pixels in less than three hours [see, for example, D. L. Howard et al. “High-Definition X-ray Fluorescence Elemental Mapping of Paintings” Analytical Chemistry vol. 84 (2012), pp. 3278-3826]. Those capabilities are achieved with several recent technological developments in high brightness synchrotron x-ray sources, high performance x-ray focusing optics, and efficient energy resolving x-ray detectors with high count rates.
Several of these synchrotron developments have also been adapted to smaller laboratory systems in the past decade, and XRF instruments have been deployed in a variety of applications, e.g. screening lead in toys and electronics [see K. Janssens et al., “Recent trends in quantitative aspects of microscopic X-ray fluorescence analysis.” TrAC Trends in Analytical Chemistry vol. 29.6 (2010), pp. 464-478], inspection of sulfur in fuel [see Z. W. Chen et al. “Advance in detection of low sulfur content by wavelength dispersive XRF”, Proceedings of the ISA (2002)], and mineral mapping in mining samples [see J. M. Davis et al., “Bridging the micro-to-macro gap: a new application for micro x-ray fluorescence.” Microscopy and Microanalysis vol. 17 (2011), pp. 410-417].
For this reason, a number of laboratory microXRF systems have also been recently developed and commercialized by the companies Bruker Corp. of Billerica, Mass., Horiba of Kyoto, Japan, and Rigaku Corp. of Tokyo, Japan.
However, the sensitivity and spatial resolution of these laboratory systems has remained limited. Very significant enhancements are required to realize a laboratory XRF with high performance for in-line applications, biological applications, or rapid mapping required for a large number of applications.
General XRF Operation.
For the XRF system as illustrated in FIG. 4, the signal and resolution are governed by the physics of the x-ray optics. The higher the x-ray flux at the sample, the larger the fluorescence signal will be. The useable flux of x-rays at the sample is given byF0∝BSsη(NA)2  [Eqn. 2]where βS is the brightness of the source, s represents the area of the x-ray source, η represents the efficiency of the optical system in collecting and refocusing x-ray photons, and NA represents the numerical aperture of the x-ray optics. Therefore, from Eqn. 2, systems with a large source size s and large numerical aperture NA along with high brightness BS are desired for high flux and therefore a good signal-to-noise ratio for the x-ray fluorescence excited by the incident x-rays.
However, the brightness BS is in turn related to the source size byBS∝1/√{square root over (S)}×  [Eqn. 3]This means that smaller sources lead to higher brightness. The effective source size can be limited by the angular width Δθ of the x-ray optic at a point on the optic surface, such as the critical angle of a reflective optic or the Darwin width if a crystal or multilayer optic is used, and will also be related to other geometric properties of the system bys≦ΔθLO  [Eqn. 4]where LO is the distance from the source to the x-ray optics. When the x-ray source size is larger than Δθ·LO, x-rays generated from a fraction of the source area may be collected by the x-ray optics while x-rays generated by the remaining fraction of the source may not be collected by the x-ray optic. Therefore, a smaller source is generally preferred to obtain high x-ray source brightness and possibly greater flux for a given x-ray optic and distance LO. However, trying to drive too much electron energy into too small a spot on the x-ray target can lead to material damage, limiting the brightness achievable.
X-ray fluorescence is often used to examine the atomic composition of materials, and for many applications, knowing the composition of various ores and complex minerals on the scale of a micron or smaller may be very useful. To achieve this, the x-rays need to be focused to a spot as small as, or smaller than, 1 micron. However, the optical system needed focus tightly and achieve high flux density at the sample can be difficult to achieve.
A limitation for such an optical system arises from the poor reflectivity of most materials at most angles of incidence. Because most materials only weakly interact with x-rays, the refractive index of a material at x-ray wavelengths may be represented by:n=1−δ+iβ  [Eqn. 5]where δ represents the dispersion and β represents the absorption. For most materials at x-ray wavelengths, the perturbations δ and β are on the order of ±10−4 or smaller, and refraction and absorption are very weak. This makes the fabrication of practical refractive lenses, analogous to optical lenses, very difficult.
However, at grazing angles, total external reflection can occur, and optics that can focus or collimate at higher efficiency for at least a portion of the x-rays can be designed. This is illustrated in FIG. 5 and FIG. 6. For an x-ray of incident at an angle θ onto a surface of a material with atomic number Z, as shown in FIG. 5, the reflectivity is nearly 100% for near-grazing angles (e.g. θ≈0°), and falls off for angles larger than a material-dependent critical angle θc, as illustrated in FIG. 6. The value of θc is given by:θc≈√{square root over (2δ)}  [Eqn. 6]which can be approximated by
                              θ          c                =                                            2              ⁢              δ                                =                                    λ                              2                ⁢                π                                      ⁢                                          4                ⁢                πκρ                ⁢                                                                  ⁢                                  r                  0                                                                                        [                  Eqn          .                                          ⁢          7                ]            where λ is the x-ray wavelength in nm, ρ is the density of the material in g/cm3, κ is a constant to convert density to the correct units, and r0=2.82×10−6 nm, the “classic electron radius” [this derivation may be found in Chapter 3, section 3.1 on “Refraction and Phase Shift in Scattering”, in Jens Als-Nielsen and Des McMorrow, Elements of Modern X-ray Physics (John Wiley & Sons, 2011)].
Using
                              λ          ⁡                      [            nm            ]                          =                  1.2398                      E            ⁡                          [              keV              ]                                                          [                  Eqn          .                                          ⁢          8                ]            this becomes
                                          θ            c                    ≈                      1.2398            ⁢                                                            κ                  ⁢                                                                          ⁢                                      r                    0                                                  π                                      ⁢                                                            ρ                  ⁡                                      [                                          g                      ⁢                                              /                                            ⁢                                              cm                        3                                                              ]                                                                              E                ⁡                                  [                  keV                  ]                                                                    =                              K            ⁢                                          ρ                ⁡                                  [                                      g                    /                                          cm                      3                                                        ]                                                                          E            ⁡                          [              keV              ]                                                          [                  Eqn          .                                          ⁢          9                ]            An empirical fit of θc for 34 elements gives an average value of K=18.9, but a better fit is achieved using K=19.7 for E<4 keV, K=19.0 for 4 keV≦E<10 keV, and K=18.4 for E≧10 keV. A Table of θc for several materials calculated using the website purple.ipmt-hpm.ac.ru/xcalc/xcalc_mysgl/ref_index.phpis shown in Table I. Even for the range of conditions here, total external reflection only occurs for grazing incidence, with angles mostly smaller than 1°, limiting the acceptance angle for most configurations.
Aside from the practical limitations on the amount of x-rays that can be collected and focused by the optical system, the major practical limitation in x-ray source brightness is limitation of the electron density and electron power incident on the x-ray target to prevent target melting or evaporation. Various target designs that incorporate cooling systems, such as water cooling channels or thermoelectric (Peltier) coolers, or using mechanical motion (such as rotating target anodes to distribute the heat deposition over a
TABLE ICritical angle for several materials and several x-ray energies.θcθcδ(mrad)(degrees)Carbon C (Diamond): ρ = 3.5 g/cm3 2.835 keV9.22E−0513.5770.778 8.048 keV1.13E−054.7490.27217.480 keV2.38E−062.1840.12530.000 keV8.09E−071.2720.07350.000 keV2.91E−070.7630.044Silicon: ρ = 2.32 g/cm3 2.835 keV6.04E−0510.9900.630 8.048 keV7.58E−063.8920.22317.480 keV1.59E−061.7810.10230.000 keV5.36E−071.0360.05950.000 keV1.93E−070.6210.036Silica (SiO2): ρ = 2.65 g/cm3 2.835 keV5.78E−0510.7520.616 8.048 keV7.13E−063.7750.21617.480 keV1.50E−061.7310.09930.000 keV5.07E−071.0070.05850.000 keV1.82E−070.6040.035Copper (Cu): ρ = 8.96 g/cm3 2.835 keV2.11E−0420.5551.178 8.048 keV2.44E−056.9820.40017.480 keV5.61E−063.3490.19230.000 keV1.90E−061.9490.11250.000 keV6.80E−071.1660.067Silver (Ag): ρ = 10.49 g/cm3 2.835 keV1.97E−0419.8321.136 8.048 keV2.94E−057.6660.43917.480 keV6.09E−063.4900.20030.000 keV2.07E−062.0350.11750.000 keV7.61E−071.2330.071Gold (Au): ρ = 19.30 g/cm3 2.835 keV2.83E−0423.8001.364 8.048 keV4.60E−059.5920.55017.480 keV1.00E−054.4800.25730.000 keV3.47E−062.6340.15150.000 keV1.24E−061.5730.090larger area) have been designed, but are still limited in the amount of brightness and therefore x-ray flux that can be achieved.
There is therefore a need for a XRF system with a compact, high-brightness x-ray source that can be focused to a small spot for XRF analysis from several hundred microns down to the scale of 1 micron or smaller.